1. Technical Field
The present disclosure relates to a coating thickness measuring instrument and, in particular, to a coating thickness measuring instrument employing an inductive probe.
2. Related Art
Conventional coating thickness measuring instruments having an inductive probe only allow thickness measurements to be made in a single limited range. A user therefore needs to select an instrument having a suitable measurement capability for his needs. If the user wishes to make measurements over a greater range than can be accommodated by a single instrument he must buy one or more additional instruments with different ranges, or at least one or more probes for a single instrument.
Having to use more than one instrument or probe is expensive and inconvenient. The additional cost is particularly difficult to justify where a user only occasionally wishes to make measurements outside a range catered for by a single instrument or probe.
It is an object of the present disclosure to address this problem.
According to a first aspect of the present disclosure there is provided a coating thickness measuring instrument having a first mode of operation in which it is operative to make measurements with a first resolution and a second mode of operation in which it is operative to make measurements with a second resolution, the first resolution being greater than the second.
Provision of two such modes of operation enables the range of measurement of a given probe or instrument to be extended whilst maximizing the available resolution of measurement. This will become more apparent from the following description.
When in the first mode the instrument is preferably operative to make measurements in a first range and when in the second mode, in a second range.
The second range is preferably wider than the first in which case the instrument provides a first, short range high resolution mode, and a second long range low resolution mode. The first and second ranges may be contiguous, overlap, or the first range may be contained within the second range. It will be appreciated that where the two ranges overlap or where one contains the other measurements may be taken in the common portion of the ranges in either mode of operation, to obtain measurements of different resolution.
The instrument preferably comprises an inductive probe comprising a drive coil and a pickup coil, more preferably two pickup coils. The probe is preferably detachable to allow other probes to be used with the instrument. The instrument can preferably operate in either the short or long range mode with a single probe.
The instrument preferably comprises a means to drive an alternating current of substantially constant amplitude in the drive coil. This may comprise an oscillator and associated control loop circuit arranged to control the oscillator in dependence upon the current flowing in the drive coil. Further, there is preferably provided a means for varying the amplitude of the alternating current in the drive coil. This may comprise a digitally controlled potentiometer. This allows a desired amplitude to be set, then maintained by the control loop.
There is also preferably provided means for sensing the variation in coupling between the drive and pickup coils and converting this to a thickness value. This may comprise a differential amplifier, means for rectifying the output of the pickup coils and an analog to digital converter. The means for rectifying may comprise a synchronous detector and may also comprise a low pass filter. The synchronous detector may be controlled by a synchronizing signal derived from the means to drive an alternating current in the drive coil. The synchronizing signal may be phase shifted relative to the alternating current in the drive coil.
In order to provide for the second range mode a means is preferably provided to modify the amplitude of the current flowing in the drive coil in dependence upon the output from the pickup coils. This means may comprise a control loop which is preferably arranged to reduce the amplitude of the current supplied to the drive coil as the differential output of the pickup coils increases. A switch is preferably provided to enable the control loop to be switched in and out of operation, in order to switch the instrument between the first and second modes. This has the effect of flattening the response of the instrument as the probe approaches a suitable substrate, extending the range of the instrument, but reducing its resolution. Thus, the resolution of the instrument is lower in the second than first range mode, at least for measurements of a similar magnitude.
The instrument preferably includes a microprocessor. The microprocessor is preferably operative to control the amplitude of the current in the drive coil, the phase difference between the current in the drive coil and the synchronizing signal and the switch for switching in the control loop arranged to modify the amplitude of the current flowing in the drive coil.
The microprocessor is preferably also operative to generate a coating thickness value from the output of the analog to digital converter using a look-up table. The instrument preferably also includes a memory, which may be comprised in the microprocessor, for storing look-up tables for both long and short range mode operation. The memory may also store look-up tables for other probes. Each look-up table is preferably associated with a particular probe and mode of operation.
The microprocessor may switch in the second control loop in or out of operation at the selection of the user, or automatically upon independence of a thickness value detected. The instrument may include a user operable control, and a means for outputting information to a user, for example a display.
According to a second aspect of the present disclosure there is provided a coating thickness measuring instrument comprising an inductive probe having a drive coil and a pickup coil, a means for driving an alternating current in the drive coil, a means for detecting the output of the pickup coil and a means for modifying the current in the drive coil in dependence upon the output of the pickup coil.
Provision of a means for modifying the current in the drive coil extends the range of the instrument by allowing the gain of the instrument to be increased but preventing saturation when higher pickup coil outputs are experienced.
The means for modifying the current in the drive coil preferably comprises a control loop and is preferably switchable in and out of operation to provide two modes of operation for the instrument. The means for driving a current in the drive coil preferably also comprises a control loop, arranged to maintain the amplitude of the current in the drive coil at a substantially constant level. Means may be provided for setting and varying this constant level.
The means for driving may comprise an amplitude controlled oscillator with the control loop being implemented by a current to voltage rectifier, a low pass filter and an error amplifier. The means for modifying is preferably arranged to modify the input to the error amplifier and hence the amplitude of the current in the drive coil.
The means for detecting the output of the pickup coil may comprise a synchronous detector.